In the manufacture of semiconductors, semiconductor devices are produced on thin disk-like substrates. Generally, each substrate contains a plurality of semiconductor devices. The exact number of semiconductor devices that can be produced on any single substrate depends both on the size of the substrate and the size of the semiconductor devices being produced thereon. As semiconductor devices have been becoming more and more miniaturized, the number of semiconductor devices capable of being produced for a given area increases. Thus, maximizing the useable surface area of a substrate becomes increasingly important.
In producing semiconductor devices, substrates are subjected to a multitude of processing steps before a viable end product can be produced. These processing steps include: chemical-etching, wafer grinding, photoresist stripping, and masking. These steps often require that each substrate undergo many cycles of cleaning, rinsing, and drying during processing so that particles that may contaminate and cause devices to fail are removed from the substrates. However, these rinsing and drying steps can introduce additional problems in and of themselves.
One major problem is the failure of the drying step to completely remove liquid from the substrates after rinsing (or any other processing step where the substrate is exposed to a liquid). If substrates are not dried properly, watermarks, which may contain small contaminating particles, may form on the surface, which can result in a drop in the yield of properly functioning devices and adversely affect the electrical characteristics of these devices. In fact, it is well known in the art that semiconductor devices produced from an area of the substrate where liquid droplets remain have a greater likelihood of failing. Thus, in order to increase the yield of properly functioning devices per substrate, it is imperative that all or substantially all liquid be removed from the substrate surface as completely as possible.
One well known method of drying semiconductor wafers is to utilize a drying vapor in combination with the liquid to be removed from the wafers. This drying process is commonly referred to throughout the art as “Marangoni Drying.” “Marangoni Drying” utilizes the phenomena of surface tension gradient (“STG”) to pull liquid from the surface of a wafer rather than allowing the liquid to evaporate. Removing liquid by evaporation is undesirable because the evaporated liquid tends to leave watermarks and residue/contaminants on the surface of the wafer.
During a conventional batch Marangoni Drying process, a plurality of substrates are immersed in a bath of liquid. A drying fluid, such as isopropyl alcohol (“IPA”), is provided atop the liquid bath. Because the IPA is miscible with the liquid, a meniscus forms as the liquid is drained past the substrates. The drying fluid is absorbed along the surface of the liquid, with the concentration of the absorbed vapor being higher at the tip of the meniscus than in the bulk of the liquid. The increased concentration of absorbed vapor results in the surface tension being lower at the tip of the meniscus than in the bulk of the liquid. This differential in surface tension causes the liquid to flow from the meniscus toward the bulk bath liquid as the substrates are withdrawn from the liquid bath. Such a flow is known as a “Marangoni” flow. This drying results in improved drying of substrates, eliminating watermarks and/or other contaminants on the substrate.
Recently, methods and systems for processing single substrates have become widely used. An example of a single-wafer cleaning system is disclosed in U.S. Pat. No. 6,039,059 to Bran, which issued on Mar. 21, 2000, the entirety of which is hereby incorporated by reference. Additionally, European Patent Application Publication EP0905747A1, to IMEC, which published on Mar. 31, 1999, the entirety of which is hereby incorporated by reference, discloses a single wafer drying apparatus that utilizes the Marangoni drying effect on a horizontally oriented rotating substrate (hereinafter referred to as “Rotagoni”).
During a Rotagoni drying process, a liquid and drying fluid are applied to the surface of a substrate. More specifically, a dryer assembly that contains a DIW supply nozzle and an N2/IPA supply nozzle is positioned above the surface of the substrate. Typically, both nozzles use a ⅛″ PFA tube installed on the dryer assembly. The DIW nozzle is installed at approximately a 45° angle to the surface of the substrate while the N2/IPA vapor nozzle is installed vertically to the surface of the substrate.
The drying assembly is swept from the substrate center to the substrate edge while the substrate is spinning. DIW and N2/IPA vapor are applied through the nozzles during the sweeping process. The DIW nozzle is leads the N2/IPA nozzle during the sweeping motion. The application of DIW rinses the substrate and keeps the substrate uniformly wet before being dried, thereby minimizing unwanted drying/evaporating on the substrate surface. The trailing N2/IPA nozzle supplies N2/IPA vapor in order to dry the wafer through STG. As a result of the IPA dissolving into the DIW, the N2/IPA drying vapor reduces the surface tension of the DIW at the IPA/DIW boundary, thereby creating the Marangoni effect and reducing the tendency of the DIW to adhere to the substrate surface. The reduction in the tendency of the liquid to remain on the substrate surface minimizes unwanted evaporation because the DIW does not remain on the surface of the substrate long enough to evaporate.
The DIW applied to the substrate is pulled radially outward by the centrifugal force of the rotating substrate, pushed away by the convective force of the N2/IPA vapor, and pulled by the STG effect formed by the IPA dissolving in the DIW at the IPA/DIW boundary. Continued rotation of the substrate combined with the continued outward sweeping of the dryer assembly, ultimately pulls the DIW off the entirety of the substrate. Therefore, the amount of residue left on the substrate is reduced.
Referring now to FIGS. 1A and 1B, a prior art Rotagoni drying system 1 is illustrated. The prior art Rotagoni drying system comprises a dryer assembly 2 and arm 3. The dryer assembly 2 is positioned above a substrate 50 to be dried by the arm 3, which supports the dryer assembly 2 in a cantilevered fashion. During a Rotagoni drying process, the dryer assembly 2 is moved in the direction indicated by the arrow 7, which is generally parallel to the upper surface of the substrate 50 in a radially outward direction.
The dryer assembly 2 has first and second N2/IPA vapor nozzles 5a, 5b extending from its housing. The dryer assembly 2 also comprises a DIW nozzle 4 coupled to the housing. The N2/IPA nozzles 5a, 5b are aligned substantially perpendicular to the upper surface 51 of the substrate 50. The N2/IPA nozzles 5a, 5b are both ⅛ inch tubes and are separated by about 1 inch. The DIW nozzle 4 is oriented at an approximately 45° angle to the substrate's 50 upper surface.
Referring to FIG. 2, a prior art Rotagoni drying method using the dryer assembly 2 of FIG. 1A and AB is schematically illustrated. As the dryer assembly 2 is moved in direction 7, the DIW wetted area 8 becomes smaller as the dried area 9 becomes larger. The direction 7 is a movement radially outward from the center of the substrate 50. The STG effect is achieved by the IPA dissolving in the DIW at the IPA/DIW boundary 13.
Another prior art Rotagoni drying system and method is disclosed in U.S. Publication Number 2004/0020512 to Hosack et al., published Feb. 5, 2004, the entirety of which is hereby incorporated by reference.
As will be discussed below, existing Rotagoni drying systems and methods are less than optimal and suffer from a number of deficiencies, including the production of watermarks, long process times, decreases in device yield, and inadequate liquid removal, especially at the edge of the substrate.